Optical fiber is now used in a variety of telecommunication applications because of its small physical size and high bandwidth capacity.
An optical fiber access network provides for the distribution of telecommunications data among various locations, such as between a central office (CO) and a device at a location remote from the CO, often called an optical network unit (ONU), over optical fibers.
In many current optical access networks, the active components in the CO, which include optical and electrical devices, are electrically powered by the power that a power utility supplies directly to the building or facility housing the CO. The ONU likewise requires electrical power for converting optical signals to electrical signals for further processing and distribution and for converting electrical signals to optical signals for transmission back through the fiber network to the CO. The magnitude of the electrical current required by each ONU is normally in the range from 0.2 to 0.6 amperes and normally, the voltage magnitude for proper operation is in the range from 70-115 volts. This power can originate from the same source in the CO, or more often, originate from a power source located remotely from the CO or another ONU. This remote power source (RPS) typically converts AC power supplied by the power utility to a lower voltage DC power suitable for handling by communications craftpersons.
The most common method of carrying the power from the CO or RPS to the ONU is via a standard copper twisted-pair telephone cable or a standard coaxial cable, neither of which contains optical fiber. In addition, it has been proposed to carry the power by using a composite cable including groups of twisted-pair telephone wires bundled together in some fashion with a plastic tube or tubes containing optical fibers. See U.S. Pat. No. 5,268,971, incorporated by reference herein.
However, composite cables, such as those described in the '971 patent are unsatisfactory in terms of their size, scalability, maneuverability and taut-sheath accessibility. Conventional composite cables contain electrical conductors arranged as twisted pairs or bundles and have a large diameter. Two wires which are twisted can require more space than the same wires which are untwisted and grouped, and even more space than the same wires which are untwisted and arranged as a radial layer in a cable. See U.S. application Ser. No. 09/108,248, filed on Jun. 30, 1998, assigned to the assignee of this application and incorporated by reference herein. The contribution of the electrical conductors to the size of the composite cable limits the scalability of the cable design with respect to the number of optical fibers and electrical conductors which can be included in a cable, because the size of a cable utilized in optical fiber networks must satisfy present standards as to duct sizes, splice enclosures, entrance ports, installation equipment and termination hardware. Also, a composite cable which has a large diameter is extremely bulky and can be heavy and, thus, hard to maneuver in storage and installation. In addition, conventional composite cables are not constructed to allow for ease of mid-span or taut-sheath access to the optical fibers without damage to the electrical conductors when the electrical conductors surround the optical fibers in the composite cable.
Furthermore, the need for twisting the telephone wires when they are used for power distribution is disappearing in modem fiber access networks because of an increased confidence in the reliability of the fiber network as the only communications medium and a decreased interest in having communication-grade twisted-pairs available for future use.
In optical fiber networks which include optical and electrical connections between prior art composite cables and ONUs of the networks, the composite cables typically include one or more pairs of electrical conductors of a small gauge, such as about 19-24 AWG. To keep the cable diameter small, each conductor pair of the prior art composite cable was designed to convey electrical energy sufficient to electrically power only one of the ONUs to which an optical fiber of the cable was to be coupled and provide optical signal service, i.e., receive optical signals from or transmit optical signals to the ONU. In such composition cables, the size or gauge of the conductors of the conductor pairs was, in part, selected so that the resistance of the conductors would not cause an excessive voltage drop between the electrical power source, which normally supplies power at 130 volts, and the ONU. Wires of small gauge could be used because the current requirement of one ONU, and hence, the voltage drop between the electrical power source and the ONU, was relatively small. Generally speaking, when the wire gauge was selected so that the voltage drop did not exceed a permissible amount, the electrical energy to be conveyed over the conductor pair would not overheat the wire to a temperature which can damage other cable components and adversely affect the optical transmission characteristics of the cable, i.e. the conductor wire had a safe current carrying capacity.
As is known, both the heating and the voltage drop are dependent upon the resistance of the conductor and upon the current magnitude, and the voltage drop also depends upon the length of the conductor which can be thousands of feet, between the source and the ONU. Since the volume resistivity of a conductor depends upon the metal or metal alloy used, the remaining variable for controlling heating is the conductor size or gage. In the prior art, the conductor size or gauge usually was selected so as not to exceed the permissible voltage drop, the heating in the expected length of conductor and the current magnitude in the worst case in field installation. By so selecting the conductor size, only one conductor size was needed for all expected installations.
Thus, the conductors of a conductor pair in the prior art composite cables were designed to have a predetermined resistance R based on the formula: EQU R=p1/(.pi.d.sup.2 /4)
where p is the volume resistivity of a conductor, d is the diameter of the conductor and 1 is the length of the conductor.
The current density (j) in a conductor is set forth by the relationship: EQU j=I/A
where I is the current magnitude and A is the cross-sectional area of the conductor. Since current density determines the conductor heating, it is also apparent that if the heating is to remain constant and at a safe level when the current is increased, the cross-sectional area of the conductor must be increased, e.g. if the current is increased three times, the area must be increased three times. Assuming that the conductor is circular in cross-section, the area A increases with the square of the conductor radius so that the area of the conductor increases rapidly with increases in conductor diameter.
A further problem with the prior practice is that the electrical conductors were cut at a point along the cable near the ONU. The conductors, after processing, were secured to the ONU terminals which normally are of a size which can receive only small size wires, e.g. of 19-24 gage. If the free lengths of the conductors were not sufficient to reach the terminals, splices were required.
Although the conductor pairs within a composite cable can be arranged to minimize the increase of the overall cable diameter (see said U.S. application Ser. No. 09/108,248), high fiber count cables would require larger numbers of such conductor pairs to provide that the ONUs which are optically served by the fibers in the cable also can be separately electrically powered by conductors of the cable. It is known, however, that increasing the number of conductors in the composite cable can limit scalability in terms of optical fiber and electrical conductor capacity and also cause manufacturing difficulties by increasing the number of adjustments to the stranding positions of conventional conductor stranding equipment which must be made when the conductors are stranded on the cable, as is commonly performed. Also, for a composite cable with a larger number of conductors, the accessibility to the conductors and any optical fibers underlying the conductors is adversely affected. The difficulty of identifying a pair of the conductors to be coupled to an ONU and the corresponding optical fibers which are for coupling to the same ONU is increased. Further, the procedure for cutting the desired conductors for electrically coupling them to an ONU and then terminating the cut conductors at the cable is more complex, especially where several ONUs are to be supplied with electrical energy from the cable at or near the same location on the cable.
Some current composite cable designs include two separate cables which are enclosed within a sheath, where one of the cables conveys only optical signals and the other cable conveys only electrical energy to electrically power all ONU s to which the optical fibers in the one cable are to supply optical signal service. These composite cable designs are extremely impractical, especially for cables having a high optical fiber count or high bandwidth optical fibers which are under development and becoming more prevalent, because the available overall cable diameter is inefficiently used, thereby limiting the scalability of such cables. Cables of this type also have limited flexibility.
Therefore, there exists a need for a composite cable which is compact, has a small diameter, is lightweight, mechanically protects the optical fibers from damage, is scalable in terms of optical fiber and electrical conductor capacity, allows for ease of mid-span or taut-sheath fiber access without harm to either the fibers or the conductors, optimizes use of overall cable diameter for conveying electrical energy, eases identification of the conductors and the corresponding optical fibers or optical fiber carrying elements which are for coupling to the same ONUs, simplifies coupling of the conductors and the corresponding optical fiber or fiber elements to an ONU during an installation and is compatible with modem optical access network limitations and standards.